1. Field of the Invention
The present invention relates to apparatus for sensing rotation of a disk and is particularly suitable for counting the rotations of a Ferraris disk in an electricity meter.
2. Description of Related Art
A watthour meter has a Ferraris disk which rotates in one sense at a rate dependent upon the power being consumed. If power is being delivered rather than consumed, the disk would rotate in the opposite sense. However in a normal domestic situation rotation in the opposite sense is more likely to be due to tampering. In a typical domestic meter rotations of the disk are counted mechanically and the cumulative number of rotations is shown on a series of displays. In order to provide for remote meter reading it is necessary to produce and store electrical signals representing a count of the number of rotations since the last meter reading was taken. This count can be stored in memory and cumulated until the meter is read.
The Ferraris disk of a conventional electricity meter has a shiny reflective surface and is provided with a broad black stripe on one face. It is desirable to use this existing marking as the basis for any sensor. It is known to use an optical method for counting rotations of such a marked disk. For example, U.S. Pat. No. 5,130,641 (Cornwall et al assigned to AMRplus Partners) and U.S. Pat. No. 4,204,115 (Boldridge assigned to Conversational Systems) both describe systems in which a light source is focused on markings on the edge of a disk with a detector aligned to receive light reflected from the edge of the disk when markings are not present.
A number of technical problems are associated with this type of optical sensor.
These include:
Variations in ambient light can affect the performance of the sensor. PA1 Eddy currents induced in the disk can cause it to jitter and vibrate when no power is being consumed, particularly when consumption ceases. If such vibrations occur when a marking is close to the sensor, false counts may be detected. PA1 Because the sensor is normally retro fitted to an existing meter, it is desirable to have simple optics, whereas some of the existing systems would require an elaborate alignment or focusing system which may make installation difficult.
In order to provide bi-directional counting, it has been proposed to use systems with spaced detectors which detect the marking in sequence. For example, such systems are described by Marshall in U.S. Pat. No. 4,321,531 and Avins in U.S. Pat. No. 4,328,463. These sensors are primarily for use in applications where the disk may be continuously rotating in one or other direction rather than for dealing with the technical problem of vibrations and jitter needed for a domestic meter which is designed to measure electricity consumption only.
The apparatus described in WO-A-94/28428 (Itron) comprises two sources of electromagnetic energy which are preferably infra-red light emitting diodes together with a single photo-diode sensitive to the infra-red pulses reflected from the surface of the disk. The photo-diode is located between the two LEDs, which are carefully focused in order to reflect from different areas of the disk which are separated in the direction of rotation. Each LED is pulsed with a different sequence and the resulting signal detected by the photo-diode is processed in order to determine the number of rotations of the disk and the direction of rotation of the disk. Although Itron uses infra-red LEDs to avoid some of the problems of varying levels of ambient light, the arrangement described may be susceptible to false triggering in high ambient light levels, particularly where the outputs of each LED are at different levels which would be typical.